Storage of liquid ozone



United States Patent Q 9 No Drawing. Filed Nov. 13, 1959, Ser. No. 852,602 7 Claims. (Cl. 62-48) This invention relates to a method for storing ozone in relatively high concentrations and for recovering the ozone in substantially pure form from the storage container when ready for use.

Ozone is an endothermic compound which is dangerous to store except in very dilute mixtures with inert gases. In high concentrations the pure gas or liquid is subject to explosive decomposition or detonation in the storage vessel which may be induced by spark, mechanical shock, sudden heat, etc. It is desirable for many uses to make ozone available in high concentrations, or in substantially pure form.

In our copending application Serial No. 715,098, filed February 13, 1958, now United States Patent No. 2,928,- 529, is described a method by which pure ozone gas or high concentrations of ozone in other gases, can be stored safely in cylinders under pressures of from about one to several atmospheres for long periods of time and in concentrations which are normally dangerous. Certain features of that application are employed in the present invention and the disclosure thereof to the extent necessary for full understanding of the present invention is incorporated herein by reference.

If an explosive gas or explosive mixture of gases is subjected to conditions which will initiate a flame, an explosion can develop in the body of the gas if the flame can be maintained. If the flame can be quenched in a short distance no combustion occurs and therefore no explosion can take place. The distance in which the flame must be quenched to prevent explosion is known as the quenching distance or quenching diameter. This varies with the particular gas or gas mixture and the temperature and pressure conditions. The quenching distances of a number of combustible or explosive gas mixtures have been reported in the book Combustion, Flames and Explosions by Lewis & Elbe, Academic Press Inc., New York City, 1951, pages 4084l2. These are reported as being from 1.6 cm. to 0.02 cm., depending on the gas or gas mixture, at one atmosphere pressure and at C. It has been reported also that the quenching distance for ozone is below about microns (0.0025 cm.).

The quenching distance is dependent upon the pressure, the gas concentration and the storage temperature. The higher the pressure and/or temperature of a gas of a given concentration, the smaller the quenching distance. For a gas like ozone mixed with varying percentages of oxygen, the quenching distance increases as the volume percent of ozone in the mixture decreases.

We have found that the quenching distance for ozone is considerably greater than heretofore believed.

The present invention broadly comprises a method of storing ozone as a solution in an inert solvent in presence of packing material such as is described in our copending application.

We have found that ozone is soluble in varying degrees in a number of organic solvents which are substantially inert to the action of the ozone and which have sufficiently low vapor pressures at the temperature at which the ozone is to be separated from the solution that it can be ob Patented July 18, 1961 tained in pure or substantially pure form by flash distillation. The preferred solvents are fully halogenated, low molecular weight saturated hydrocarbons or mixtures thereof, in which the molecule contains at least one fluorine atom, the remaining substituent atoms being fluorine or another halogen such as chlorine or bromine. For practical purposes and because of their commercial availability in high purity, fluorochlorocarbons having from one to three carbon atoms in the molecule are preferred. Representative compounds together with their approximate melting points and approximate boiling points are shown in Table I.

Table 1 Approximate Approximate boiling point, 0.

1 At atmospheric pressure. 2 At atmospheric pressures.

The choice of the solvent depends upon several factors including the solubility of ozone therein. The solvent should be non-reactive with ozone under the conditions of storage and at the temperature at which the ozone is distilled from the solvent. It should not catalyze decomposition of ozone. It should have a negligible partial pressure at the temperature at which the distillation is to take place so that clean, substantially complete separation can be effected. Its melting point should be sufficiently low so that the solution can be stored at temperatures at which spontaneous, slow decomposition of ozone is negligible.

It has been found that solutions of ozone in these solvents up to concentrations of about 30% by weight can be stored safely without danger of detonation due to shock or other stimuli which would normally induce explosion in gas mixtures of that high concentration. Ozone is miscible in all proportions with some of the solvents even at atmospheric pressure, while in other cases it is less soluble, although the degree of solubility depends on the pressure.

The ozone solution is stored in ordinary fluid-tight cylinders or other vessels sufiiciently strong to Withstand pressures of several atmospheres. Some solutions can be stored in glass vessels, which is convenient for use in the laboratory where small quantities are required at any one time, and where precautions can be taken against breakage, and where damage from fumes can be prevented or minimized if breakage should occur. The vessels are provided with the usual closure-controlled discharge passages for transfer of the solutions to other vessels, or for removal of gaseous ozone directly from the storage vessel. Since it is only necessary for the cylinder to withstand a few atmospheres pressure, it is not necessary that it be of the heavy construction employed for gases such as hydrogen and oxygen, etc. The vessel should be of a material which causes little or no catalytic decomposition of ozone. Since the surface area of the vessel with which the solution is in contact is relatively small most corrosion-resistant materials can be used. For example, stainless steel or glass-lined steel cylinders are suitable. Any gaskets, packing material, etc. which are required in valves or other fittings should be resistant to the reaction of ozone. The normally solid polymers of tetrafiuo-roethylene or dichlorodifluoroethylene (Du Ponts Teflon, or Minnesota Mining and Manufacturing Companys Kel- F, respectively) are suitable for such purposes. Pipe or hose connections to the vessel should likewise be resistant to ozone. If necessary there may be jacketed for circulation of a refrigerant. If gaseous ozone is distilled directly from the vessel, the connecting lines should be protected from shock, etc., and suitable precautions against damage to equipment or injury to the operator should be taken in the event a line should burst.

The cylinder is entirely filled with packing material which is substantially inert to the ozone and provides a large number and a large total volume of free voids, the diameter of said voids being not substantially greater than the quenching distance of said gas. A suitable packing material comprises hollow spheres of substantiallq pure fused alumina which contain relatively large pores in the walls. The spheres may suitably have an average diameter of about 250 to about 300 microns. Materials suitable for use and other details are more fully described in our copending application Serial No. 715,098. These spheres prevent the propagation of flame or explosion in the event of accidental ignition of the ozone gas in the vapor space above the solution. The presence of the spheres prevents possible destructive explosion of the substantially pure ozone which accumulates in the vapor space, for example, when the solution is withdrawn from the cylinder and/or when the ozone gas is distilled from the solution and removed from the cylinder.

If the vessel were to be maintained in an upright position at all times and ozone distilled from the solvent (which is not to be removed from the cylinder), it would only be necessary to fill that part of the upper portion of the cylinder which, after complete discharge of the ozone, would comprise the vapor space in the cylinder. This could be done by positioning a supporting screen at a level below the minimum liquid level in the cylinder to support the particles in the vapor space or potential vapor space. This would, of course, allow a considerably larger volume of solution to be put in the storage vessel since only a small fraction of the liquid storage space would be occupied by the particles. However, if the solution is to be removed from the cylinder and/or if the cylinder is likely to be tilted or laid on its side it should be entirely filled wtih the particles.

The permissible storage temperature and pressure conditions for storage of the solutions of this invention, are somewhat interdependent. The higher the storage temperature, the higher the pressure on the vessel. Permissible temperature limits depend upon the nature of the solvent and its stability toward ozone. The lowest temperature of storage should be above the eutectic point of the ozone-solvent mixture in order to maintain a homogeneous liquid system. The highest temperature limit should be such that the vapor pressure of ozone does not rise above about ten atmospheres. In any case, the temperature should be kept below about 20 C. during storage so that the rate of slow, spontaneous ozone decomposition is practically zero. At temperatures of about 20 C. there is less than 2% decomposition of ozone over a period of about two months storage.

The storage temperature, as well as the distillation temperature, depends in part upon the boiling point and partial pressure exerted by the solvent. The temperature must be such that the solvent is liquid and must be sutficiently low that the solution can be raised to a point at which the ozone will distill oil without the solvent exerting a substantial partial pressure. Thus, for example, a solution of ozone in dichlorodifluoromethane must obviously be stored well below 30 C. It may be stored at, say C. When the ozone is to be distilled oil the temperature may be raised, say to Dry Ice temperature, where the vapor pressure of the solvent is still so low that the effluent ozone gas contains only a few parts per million of the solvent.

The solution may be formed in several ways; for example, by absorbing the ozone directly in the cold solvent by countercurrent absorption from a stream of ozonized oxygen. Ozone may be formed in oxygen or air by the usual known methods. The ozone-containing gas may then be passed through a packed-absorption column in countercurrent flow with the liquid solvent at a temperature and pressure conducive to formation of a solution of the desired ozone concentration. The temperature is usually at or below the contemplated storage temperature. The pressure may be somewhat higher than that at which the ozone is to be stored. Since higher pressures would tend to increase the amount of oxygen dissolved in the solvent, the solution withdrawn from the absorption column can be passed through a conventional stabilizing tower to remove the oxygen, if so desired, before charging the solution to the storage vessel.

Another method is to condense liquid ozone from ozonized air or oxygen and mix it with the solvent at or below the contemplated storage temperature. Since liquid ozone is extremely hazardous to handle, precautions against accumulation of substantial amounts of it, and against induced explosion or detonation must be employed.

As another alternative, the ozone may be first concentrated by absorption in a relatively higher boiling fluorochlorohalocarbon and then distilled from this and the ozone vapors absorbed in the solvent in which it is desired to effect the final storage.

As previously mentioned, the solution is maintained at a low temperature and at a pressure such that the vapor pressure of the ozone does not rise above the ten atmospheres. In order to recover the ozone from the solution, it is warmed to a point at which the vapor pressure of the ozone increases but below that at which the vapor pressure of the solvent becomes appreciable. As an example, difluorodiehloromethane (Du Ponts Freon-l2) has a vapor pressure at its melting point (158 C.) of about 10 mm. of mercury. Assuming an ideal solution of 50 mol percent of ozone in Freon-12 at just above the melting point, the ozone and also the total vapor pressure would be 5 mm. of mercury. A 75 mol percent ozone solution at the same temperature would have a vapor prmsure of about 7.5 mm. A stream of oxygen containing 5 mol percent of ozone would have a partial ozone pressure of about 37.5 mm. Thus, any solution up to ozone purity in Freon-12 could be prepared by absorption at this temperature.

Pure ozone can be delivered from the solution by warming it to a higher temperature. Thus, at 89 C. the partial pressure of ozone in a 50 mol percent solution in Freon-12 is 3.1 atmospheres. The partial pressure of the Freon-12 is: only 25 mm. of mercury. Thus, the composition of the evolved gas phase is 99 mol percent ozone and one mol percent of Freon-12. If the presence of this amount of Freon-12 is objectionable the last traces can be removed by passing the evolved gas through a higher boiling (i.e. lower vapor pressure) solvent such as trichloroiiuoromethane (boiling point 24 C.) to absorb the Freon-12, yielding ozone gas of about 100% purity. The absorbed dichlorodifluoromethane (boiling point 30 C.) can be recovered by fractional distillation.

'I he ozone gas can be passed directly to the point of use, or can be passed into a cylinder containing alumina spheres such as are described in our said copending application, for safe intermediate storage in the gas phase.

Another method of safe handling of the evolved pure or highly concentrated gas is to pass it to the point of use through pipes or tubes of an internal diameter equal to or less than the quenching diameter of ozone; or through larger tubes packed with particles of a size such that the diameters of the voids are equal to or less than the quenching diameter of ozone. The principles discussed here are disclosed in our copending application referred to above.

It is also possible to remove the solution from the storage container and distill out the ozone by passing the solution countercurrent to a stream of an inert gas or oxygen gas to obtain a mixture of gases of any desired concentration. It is evident that the efliuent dilute gas could be scrubbed to remove the solvent which might be carried over, using a higher boiling absorbent and then recovering the solvent by distillation as referred to above.

Another method is to introduce the solution into a distillation column, at least the upper portion of which is packed with solid particles having flame arresting characteristics (such as discussed above and in our copending application). The pure ozone is withdrawn from the top, and ozone-free solvent from the bottom.

Such a system can be used as a combined distillation and absorption column to scrub out any vaporized or entrained solvent from the ozone gas. This can be done by providing a chimney tray or pan above the point at which the solution is introduced intothe column. Cold higher boiling scrubbing solvent can be introduced as reflux at top of the column and withdrawn from the chimney tray together with any of the lower boiling solvent which has been scrubbed from the gas stream. These can then be separated by distillation as previously described.

According to another embodiment, the ozone may be partially decomposed into oxygen within the cylinder, if it is desired to obtain a mixture of ozone and oxygen. This can be done by injecting a material which catalyzes slow and controlled decomposition of the ozone preferably at or near the bottom of the container. There are various ways of doing this. For example, a capsule may be inserted into the body of the particles with which the cylinder is filled. The capsule may contain the catalyst which may be a solution or may be solid at storage temperatures. The outlet tube of the capsule may be provided with plugs of material which are solid at the storage temperature but which melt when warmed to the distillation temperature of the ozone, thereby injecting the catalyst into the ozone solution. As the ozone is slowly decomposed, a mixture of oxygen and ozone sweeps upwardly through the particles and is withdrawn through the valved outlet.

Instead of a capsule of this type a dip-tube may be provided in the cylinder through which the catalyst or reagent can be injected near the bottom of the cylinder from an outside source. Another method is to withdraw the solution from the container and effect the catalytic partial decomposition in an outside zone.

Mixtures of oxygen and ozone of any desired concentration can be obtained by proper choice of catalyst, the temperature of the reaction, etc. The catalyst should be of such a character as to promote slow and controlled decomposition. It should be of such a nature that it can remain in solution and be washed out, for example with water, aqueous alkali or other suitable material which will dissolve or destroy the catalyst. If this should be difficult, the contents of the cylinder can be discarded and fresh supply employed.

Chlorine or bromine in proportions of about 15% by weight or less dissolved in pethalogenated hydrocarbons such as heretofore discussed may be used as ozone decomposition catalysts. When the ozone is exhausted, any remaining chlorine or bromine may be stripped from the solution with a dry gas such as dry oxygen. Catalysts such as cobaltous trifluoroacetate or cobaltous salts of other perfluoro acids dissolved in the pure acids or mixtures of acids and perfluorinated solvents may be used. These can be destroyed or dissolved out of the solvent when the reaction is complete. If aqueous solutions or water is used the recovered solvent should be dried before reuse.

It will be noted from the table that the only fully chlorinated hydrocarbon which could be used is carbon tetrachloride and this is not preferred because of its relatively high melting point. However, carbon tetrachloride can be used in admixture with other lower melting materials to form intermediate melting solvents and to alter the vapor pressure characteristics and/ or solvent power. Mixtures of two or more fiuoro carbons may be used.

For carrying out certain chemical reactions in which the presence of the solvent is not objectionable (or may even be desirable) the solution of ozone in the solvent can be added directly to the reaction mixture. The solvent can be recovered later from the reaction products or if the amount of solution is small the solvent can simply be allowed to evaporate. This eliminates any hazard which is attendant the separation of ozone in pure form or in the form of concentrated mixtures in another gas. An example of this might be found in certain bleaching operations where the amount of ozone used is small. The solution could be sprayed into or onto the material to be bleached. In such a case a container equipped with an appropriate spray nozzle and dip tube or other device could be used to effect distribution as a fine mist.

We claim as our invention:

1. A method of storing ozone which comprises forming a solution thereof in a solvent inert to ozone and having a negligible vapor pressure at the storage temperature and storing the solution at a temperature below about zero C. but above the freezing point of the solution at which the rate of decomposition of ozone is slow, in a liquid-tight and vapor-tight zone filled with a catalytically inert packing material of a particle size and shape such as to provide a large number and a large total volume of free voids, the diameter of said voids being not substantially greater than the quenching diameter of ozone flame at the storage temperature and pressure.

2. The method of claim 1 wherein the solvent is a saturated completely halogenated hydrocarbon of one of three carbon atoms, said solvent having a melting point below about 20 C. and a boiling point substantially above that of ozone.

3. The method of claim 2 wherein the solvent contains at least one fluorine atom per molecule.

4. The method of claim 1 wherein the solvent is a saturated fluorochloro carbon having one to three carbon atoms per molecule.

5. A method which comprises forming a substantially saturated solution of about ten to about thirty percent ozone in a liquid solvent substantially inert to ozone at a temperature below about zero C. at which the solvent is liquid but at which it exerts a negligible partial pressure on the system, storing said solution in a confined zone, the entire vapor space of which is filled with a particulate, catalytically inert packing material of a physical shape and size as to provide a large number and a large total volume of free voids, the diameter of said voids being not substantially greater than the quenching diameter of ozone flame at the storage temperature and pressure, said ozone being stored at a temperature below about 20 C. but above the freezing point of the solution and an absolute pressure not substantially above ten atmospheres.

6. The method of claim 5 which comprises the further step of raising the temperature of the solution to a point above said storage temperature at which said ozone will boil ofi, but below that at which the solvent exerts any References Cited in the file of this patent UNITED STATES PATENTS White July 21, 1942 Hintze July 2, 1946 Haller Mar. 3, 1959 Grosse Mar. 15, 1960 

1. A METHOD OF STORING OZONE WHICH COMPRISES FORMING A SOLUTION THEREOF IN A SOLVENT INERT TO OZONE AND HAVING A NEGLIGIBLE VAPOR PRESSURE AT THE STORAGE TEMPERATURE AND STORING THE SOLUTION AT A TEMPERATURE BELOW ABOUT ZERO*C. BUT ABOVE TH FREEZING POINT OF T EH SOLUTION AT WHICH THE RATE OF DECOMPOSITION OF OZONE IS SLOW, IN A LIQUID-TIGHT AND VAPOR-TIGHT ZONE FILLED WITH A CATALYTICALLY INERT PACKING MATERIAL OF A PARTICLE SIZE AND SHAPE SUCH AS TO PROVIDE A LARGE NUMBER AND A LARGE TOTAL VOLUME OF FREE VOIDS, THE DIAMETER OF SAID VOIDS BEING NOT SUBSTANTIALLY GREATER THAN THE QUENCHING DIAMETER OF OZONE FLAME AT THE STORAGE TEMPERATURE AND PRESSURE. 